1. Field of the Invention
The invention relates generally to magnetic tunnel junction (MTJ) and particularly to spin transfer torque magnetic random access memory (STTMRAM) employing MTJ.
2. Description of the Prior Art
A typical spin transfer torque magnetic random access memory (STTMRAM) magnetic tunnel junction (MTJ) stack has a pinned reference layer whose magnetization is fixed in a certain direction by either intrinsic anisotropy field or through an exchange coupling field from an adjacent magnetic layer. The MTJ of the STTMRAM also has a switchable free layer, whose magnetization direction can be switched relative to that of the pinned reference layer when electric current flows between the pinned reference layer and the switchable free layer through a junction layer, which is typically made of an oxide of magnesium (Mg), aluminum (Al) and titanium (Ti), or a metallic layer of copper (Cu), silver (Au), or gold (Ag), where the different relative angles of the magnetization directions between the free layer and reference layer cause different resistance levels across the MTJ stack. Thus, by switching the free layer magnetization directions with the electric current, an STTMRAM can be switched into high and low resistance states.
FIG. 1 shows a typical prior art STTMRAM stack structure with in-plane magnetization in the magnetic layers. Layer 1 is a free layer whose magnetization can be switched in-plane by applying an electric current through the stack. Layer 2 is the MTJ junction layer. Layer 3 is the reference layer. Layer 4 is typically a ruthenium layer. Layer 5 is the pinned reference layer. Layer 6 is an antiferromagnetic (AFM) layer. Layer 3 and layer 5 are exchange coupled through layer 4 with antiferromagnetic orientation. The tri-layer structure made of the layers 3, 4 and 5 is generally referred to as a synthetic-anti-ferromagnetic (SAF) structure, which shows effective magnetic moment to be about 0 due to strong antiferromagnetic coupling between the layer 3 and the layer 5. Layer 6 then exchange couples to layer 5, and the SAF, and pins the SAF magnetizations in-plane.
In a well-known physics phenomenon, called “spin-pumping”, the dynamic magnetization resonance and oscillation during the switching process of a magnetic layer may create a spin current in the non-magnetic metal layer that is in direct contact with the magnetic layer. Such spin current is undesirable in the STTMRAM as it increases the effective damping of the magnetic layer, especially in the free layer of the STTMRAM, and makes switching of the magnetization by electric current more difficult.
In prior art using an oxide layer in between the magnetic free layer and the metal contact layer that leads electric current into the MTJ, the effect of spin pumping may be reduced. Such reduction is caused by the spin scattering through a non-conductive magnesium oxide (MgO) layer that quenches or dissipates the uniform spin current from the spin-pumping effect. Thus, effective damping is reduced and switching of the free layer is easier by electric current. However, the practical application of the STTMRAM MTJ stack is quite restrictive and prevents wide use of these devices for at least the following reasons.
Typical oxide material, for example magnesium oxide (MgO), is usually non-conductive therefore producing a large resistance if used with a thickness larger than 1 nano meter (nm). For spin-pumping quenching purpose, a thicker oxide layer, or more properly referred to as a spin confinement layer (SCL), which confines spin only to the magnetic layers, is generally preferred. Such preference however, presents a dilemma in using normal insulator-like oxide material because generally a thicker layer is preferred for quenching spin current but it undesirably produces too high of a resistance and low tunnel magnetoresistance (TMR) in patterned MTJs, employed in STTMRAM devices.
What is needed is a STTMRAM MTJ stack having a spin-pumping suppression layer with low resistance and suitable for practical applications.